1. Field of Invention
The present invention relates to an overvoltage protection element including a housing and having at least two terminals for electrical connection of the overvoltage protection element to current or signal paths to be protected, and with at least one arrester, including a varistor, located within the housing.
2. Description of Related Art
Electrical circuits and systems normally work without problems using the voltage specified for them, that is, the rated voltage. This does not apply when overvoltages occur. Overvoltages are considered to be all voltages which are above the upper tolerance limit of the rated voltage. They also mainly include transient overvoltages which can occur due to atmospheric discharges, but also due to switching operations or short circuits in power supply grids, and can be galvanically, inductively or capacitively coupled into electrical circuits. In order to protect electrical or electronic circuits, including electronic measurement, control and switching circuits, wherever they are being used, against transient overvoltages, overvoltage protection elements have been developed and have been known for more than 20 years.
Measurement, control and regulation engineering lines form the nerve paths of an industrial system. Their smooth operation therefore presupposes a high level of availability of the transmitted signals. The protective circuits of the corresponding overvoltage protection devices must therefore be matched to different signal and measurement principles. Arresters employed in such overvoltage protection devices can include varistors, suppressor diodes and gas-filled surge diverters or spark gaps and combinations thereof. The individual arresters can be distinguished among others by the level of the discharge capacity or the protective level. While varistors are generally used as a middle protective stage, gas-filled surge diverters and spark-gaps are generally used as coarse protection. Moreover, the individual arresters can be divided into voltage-limiting components (e.g., varistors) on the one hand and voltage-switching elements (e.g., gas-filled surge diverters and spark gaps) on the other. Varistors, as discussed herein, can include arresters, and the like.
As a result of ageing and temporary overvoltages (TOV) in the range of seconds, especially in overvoltage protection elements with a varistor as the arrester, an unwanted increase of the leakage current of the varistor at operating voltages occurs. At present, overvoltage protection elements with a varistor therefore often have a thermal disconnect device by which a varistor which is no longer serviceable is disconnected from the current path to be monitored. In known overvoltage protection elements, the state of the varistor is monitored according to the principle of a temperature switch, so that when the varistor overheats, for example, due to leakage currents, a solder connection provided between the varistor and a separating means is broken, leading to electrical disconnection of the varistor.
Such an overvoltage protection element is described in German patent application DE 695 03 743 T2, which corresponds to European patent application EP 0 716 493 A1. Such overvoltage protection element includes two varistors located parallel to one another, with a thermal disconnect device additionally connected to an optical state display, so that the state of the overvoltage protection element can be read directly on site using the optical state display. As an optical state display, such overvoltage protection element includes a first slide, which is located in the housing, and which is actuated by the separating tongues, which form the separating means, and in doing so interacts with a second slide, which can be moved relative to a viewing window, depending on the position of the first slide.
A similar overvoltage protection device with a thermal disconnect device is also described in German patent application DE 20 2004 006 227 U1, which corresponds to US patent application Publication No. 2005/0231872, wherein a varistor is used as the arrester, according to one embodiment (e.g., shown in FIG. 5). Such an overvoltage protection device consists of a bottom part, which is provided with a terminal, and an overvoltage protection element, which is made as a “protective plug” and which can be simple plugged into the bottom part of the device. In addition, such overvoltage protection device has a changeover contact, acting as the signaler for remote reporting of the state of the overvoltage protection element, and both the changeover contact and also the optical state display can be actuated via a common mechanical actuation system.
However, a disadvantage of such known overvoltage protection devices and overvoltage protection elements is that each opening contact thereof can produce an arc at operating voltages greater than 30 volts and at high current loads. Thus, when the solder connection is broken, an arc can occur between the varistor and the separating means, and which can lead to damage of components located within the overvoltage protection element, or to damage of the overvoltage protection element itself, especially to the plastic housing which surrounds the varistor. Since such overvoltage protection elements or overvoltage protection devices are often located with several adjacent to one another or to other electronic devices in a switchgear cabinet, an arc which occurs within the housing, can cause adjacent overvoltage protection devices or other electronic devices also to be destroyed or damaged.
German patent application DE 601 12 410 T2 discloses an overvoltage protection device that includes a varistor (e.g., shaped as a wafer), which is located in a metal pot-shaped housing and which is braced against the bottom of the housing using a piston-shaped electrode. The housing is closed with a cover, which is either screwed into the pot-shaped housing or is attached by a snap ring or clip, which locks in a groove in the side wall of the housing. In the cover, there is an opening through which the shaft of the electrode is routed out of the housing for electrical connection of the electrode. The second terminal for electrical connection of the overvoltage protection device to the current or signal path to be protected is formed on the housing. For electrical insulation of the electrode relative to the housing, there is provided an insulating ring, which is located within the housing and which likewise, has an opening for the shaft of the electrode.
According to another version, an overvoltage protection device described in German patent application DE 601 12 410 T2 includes two varistor wafers, which are each braced against the middle wall of the cylindrical housing using a piston-shaped electrode. On the housing, a housing electrode clip is formed to connect the housing.
Although making the housing from aluminum does prevent destruction of the housing when an arc occurs on the varistor, the making of electrical contact of the varistor and its arrangement in the housing, and the arrangement and configuration of the electrodes, especially the necessity of routing the electrodes for electrical connection through the housing cover or the end caps electrically insulated from the outside, make such a structure and installation of such an overvoltage protection device relatively complex.
Therefore, an object of the present invention is to develop an overvoltage protection element that it is better matched to thermal and dynamic loads, so that damage of the overvoltage protection element does not occur, advantageously, reducing endangerment of individuals or damage to other adjacent devices, and which can be economically built, and easily, installed.